Optical nanostructures have been the object of scientific investigation for several years but advances in material science and imprint lithography have only recently resulted in their cost effective manufacturing and availability as optical components in industry.
Optical nanostructures derived with feature sizes below the wavelength of light are known to have uniform behavior over a broad wavelength, wide acceptance angles and unique optical properties as a function of varying dimensions of their underlying grating features. The physical interaction of light with the nanostructure obeys the application of physics of diffraction gratings, but the scale of the structure causes changes in the boundary effects. As a result, quantum effects influence the classical optical effects of reflection, refraction and diffraction, resulting in nanostructure's unique optical properties. Recently, a nanostructure has been created to function as a polarizing beam splitter that performs as a perfect mirror for incident light with the light pomarized parallel to the nanostructure and a perfect window with light polarized perpendicular to the nanostructure. By comparison to traditional optics, the aforementioned nanostructured polarizing beam splitter has been demonstrated to perform with 180 degrees of beam separation in a package size under one millimeter versus two degrees of separation for a 15 millimeter length of birefringent crystal. Generally, nanostructures exhibit such unique optical properties as a result of having feature sizes in the hundreds of nanometers to tens of nanometers, below the wavelength of incident light, eliminating all high-order diffractive modes and operating exclusively on zero-order diffraction properties.
As a result of thier low cost of manufacturing, unique optical properties, high performance and miniature form factor, optical nanostructures represent a promising new technology that will have broad ramifications to tomorrow's optical systems.
Realizing the performance and value of optical nanostructures is tantamount to overcoming the primary challenge of integrating these optical structures into other optical elements. Nanostructures may be heterogeneously or monolithically integrated with other optical elements, integrated as thin-films placed adjacent to, affixed to, or inserted into other optical components such as lasers, planar lightwave circuits and liquid crystal devices. The challenge of integrating nanostructures with other optical elements and obtaining the extraordinary performance and scale benefits is a serious undertaking given that the integrated structure will carry a performance metric based on the additive sum optical properties of the two individual structures plus any distortion caused by the interface of the nanostructure and optical element. As a result, the performance of integrated structures usually do not offer the same level of high performance provided by the nanostructure alone. There is a strong need, therefore, to increase the performance of the underlying optical elements targeted for integration with sub wavelength optical elements.
Liquid crystal technology is known to be dynamically controlled and configured to enable a range of optical switching and signal conditioning applications. Formed with opposing plates of sealed glass, liquid crystal cells are considered a prospect technology and integration target capable of supplying the active layer to a nanostructure integrated therewith. Wang et. Al has recently demonstrated an experimental electrically tunable filter based on a waveguide resonant sub-wavelength nanostructure-grating filter incorporating a tuning mechanism in a thin liquid crystal. The device experiment was functional and exhibited performance of 30 nanometer tuning.
It is generally known that the performance of liquid crystal technology is susceptible to temperature and humidity change, and that high humidity and temperature changes cause decreased optical performance, resulting in high insertion loss and low extinction, two critical measures of a cell's performance.
The speed performance and optical characteristics of the liquid crystal medium as a function of applied electric field varies with temperature. In a liquid crystal cell relatively modest changes in temperature can result in relatively large changes in the transmission of light, index of refraction, and the speed of the liquid crystal state changes. FIG. 1 shows the temperature influence of a liquid crystal cell index of refraction across voltage. FIG. 2 shows that voltage for a selected transmission of light in one temperature range will provide a different transmission of light at different temperatures. FIG. 3 shows the relationship between temperature and the amount of time it takes a liquid crystal cell to change states, which decreases with increased temperature. FIG. 3 also shows that switching times of liquid crystal cells are sensitive to cell gap thickness. The series represented in the figure are two cells each having different gap sizes. More specifically, the faster switching cell has a cell gap 0.4 micron larger than the slower cell. Clearly, size and the effect of the change in optical properties are factors in controlling the optical performance in the various states of the liquid crystal cell across temperature.
In order to ensure that the temperature of the liquid crystal medium can provide stable operation and within a practical response time, prior art liquid crystal cells are known to utilize active thermal management systems based on independent temperature sensor and heater elements. JACKSON et al. relies on a resistive heating element that can be energized to heat the liquid crystal cell whenever the temperature of the cell drops below a predetermined temperature trip point. JACKSON does not accommodate feedback to the voltage control of the cell and fails to handle ambient temperature increases above the trip point. McCartney et al. provides a more complete solution that incorporates the output of the temperature sensor into a temperature feedback loop to adjust voltage in response to temperature change. In this design, a two-dimensional lookup table provides the output voltage for any temperature and pixel attribute combination. McCartney's design, however, does not scale to high resolution optical systems without increasing the size of the lookup table.
In general, the prior art liquid crystal thermal management systems rely on use of individual discreet devices for heating and sensing the liquid crystal cell. These devices are generally affixed to the outside glass of the cell at disparate locations so they are generally incapable of functioning uniformly across the cell. In addition, because these devices are usually affixed to the outside glass, all heating and sensing functions directed to the liquid crystal molecules on the inside of the glass must be translated through the glass medium. This can result in hysterises and other effects that distort the effectiveness of closed loop temperature sensing and heating systems. Finally, prior-art liquid crystal cell heaters and temperature sensors are typically attached to the cell using epoxy resins, and epoxy resins are generally known to absorb moisture in high temperatures and high humididty conditions, which leads to degradation or inconsistancy in cell performance.
The performance of liquid crystal cells are generally very sensitive to moisture and humidity. Prior art liquid crystal seals are known to provide varying levels of protection of liquid crystal cells from moisture and humidity. The prior art designs generally seal and space the cell with glass beads, frit and organic polymers such as epoxy resin. Sealing materials are generally disposed, in the form of gaskets, about the periphery of the cell. The advantage of a seal of glass frit is known to be that such seal is practically impervious to gas and vapors, but this approach requires formation by high temperature processing, and high temperature processing tends to distort the substrate and render difficult control uniformity of the distance between the inner surface of the parallel substrates. This gap (containing the liquid crystal material) must be maintained with a high degree of uniformity to achieve precise operation of a liquid crystal cell. Accurately controlling the liquid crystal cell gap is keystone to enabling high performance nanostructured liquid crystal optical systems of the present invention.
In producing an effective glass frit seal, the frit is generally applied to a surface of one of the substrates as a paste of glass powder particles dispersed in a liquid vehicle. The substrate is subsequently heated over a programmed temperature regime wherein, at lower temperatures, the solvent is evaporated and the binder is burned off, and hence in the higher temperature portions of the regime, the glass powder itself melts and coalesces to form a strongly adhesive bond to the glass substrate. Subsequently, the second glass substrate is positioned over the coalesced frit and the entire assembly is again subjected to a programmed temperature regime during which the temperature is raised within a few tens of degrees of the glazing temperature of the glass frit. At this relatively high temperature, the glass frit wets the second substrate to acquire satisfactory adhesion thereto. It is known that this second heating cycle tends to soften the substrates and cause warpage thereof, with the result that cells, particularly those of larger surface area, sealed by this glass frit method tend to have a very low percentage of acceptable manufacture.
It is generally known that warpage during fabrication can be prevented by the alternate use of organic polymer sealants, such as epoxy resins and the like, which can be processed at much lower temperatures. Polymer sealants may be screen printed from a solution or dispersion of the polymer in a solvent, or a polymer sheet can be cut into the shape of a gasket which is sandwiched between the substrates to be sealed, and the sandwich is subsequently heated to effect such seal. It is also known to introduce the polymer along the edges of an assembly of two substrates which are kept otherwise separated by interior spacers. However, such organic polymer sealants have a relatively high permeability to water vapor. Under high temperature and humidity conditions, water vapor permeates into the seal causing the expansion of the seal and a shape change in the liquid crystal cavity that results in a change in the known performance of the liquid crystal cell.